Hydrogels comprise water and polymers and are useful for medical and pharmaceutical applications (e.g. see Peppas, N. A.; Editor, Hydrogels in Medicine and Pharmacy, Vol. 3: Properties and Applications. CRC Press Boca Raton 1987; p 195 pp.). Hydrogels are usually held together via physical or chemical crosslinks, otherwise the polymers of which they are comprised would dissolve in the solvent (water). Polyelectrolyte complexes are interpenetrating complexes of one or more predominantly positive polyelectrolytes and one or more predominantly negative polyelectrolytes. The opposite charges on the polymers form ion pairs between chains, holding the chains together. This ion pairing is a type of physical crosslinking. Polyelectrolyte complexes in contact with aqueous solutions can be considered hydrogels with high crosslinking density.
Recent studies have evaluated the static mechanical properties of polyelectrolyte multilayers, which are ultrathin films of complexed polyelectrolytes. See, for example, Jaber, J. A. and Schlenoff, J. B., J. Am. Chem. Soc. 128, 2940-2947 (2006). Polyelectrolyte multilayers are intermolecular blends of positively and negatively charged polyelectrolyte, wherein each layer of polyelectrolyte added to a growing film has an opportunity to complex efficiently and completely with the existing material, excluding the maximum amount of water. The elastic modulus of these films ranges from kPa to MPa. However, these films are far too thin (a few micrometers or less) to be used for mechanical components in most systems. Furthermore, little is known of the dynamic mechanical properties of molecularly blended complexes of positive and negative polyelectrolytes.
There is a need to prepare articles with dimensions in the millimeter to centimeter to meter scale to provide materials and shapes for biomedical and engineering applications. Polyelectrolyte complexes are prepared in a straightforward manner by mixing two or more solutions of positive and negative polyelectrolytes. However, the resulting precipitate is gelatinous and difficult to process. The dried complexes, for example, are generally infusible and therefore cannot be injection molded or formed into articles under elevated temperatures. Michaels (U.S. Pat. No. 3,324,068) has disclosed the used of non-volatile plasticizers such as nonvolatile acids, organic oxysulfur compounds and organic oxyphosphorous compounds to decrease the brittleness of nonporous polyelectrolyte complexes when they are dried. U.S. Pat. No. 3,546,142 describes a method for creating solutions of polyelectrolyte complexes using aggressive ternary solvents which are mixtures of salt, water and organic solvent. Said solutions of complexes may be formed into nonporous solids by diluting the solution, or by evaporating the solvent (film casting). Mani et al. (U.S. Pat. No. 4,539,373) point out that the solid complexes “are not thermoplastic, i.e. they are not moldable or extrudable, so they must be handled as solutions.” Mani et al. disclose a nonporous polyelectrolyte complex comprising thermoplastic repeat units which can be thermally molded.
One of the many biomedical applications of hydrogels is as materials for wound protection or dressing. However, the brittle nature of polyelectrolyte complexes prevents them from conforming efficiently to the wound or limb when they are used for wound dressing. Effing (U.S. Pat. No. 6,936,746) recognized that “absorbers of this kind on a polyelectrolyte base are hard and brittle because of their high salt content and their crosslinking in the solid state.” Thus, U.S. Pat. No. 6,936,746 discloses a wound dressing comprising fibers and polyelectrolyte complexes which has adequate mechanical stability. In order to impart supporting or load-bearing properties to the dressing the fibrous complex had to be further applied to carrier materials.
U.S. Pub. No. 2009/0162640, which is incorporated fully by reference, describes fully hydrated (i.e. complexes in contact with water) polyelectrolyte complexes may be formed into shapes without the addition of organic solvent, and without the need for dissolution, if they are doped with salt ions to a sufficient extent.
In many cases, porosity within a gel is desired. Said pores are of micrometers in diameter. Open shell pores are not isolated from each other by a shell wall and are thus interconnected. Liquid may flow freely from one pore to another. In the open-shell pore architecture, liquid may flow through an entire article without having to traverse solid material. Open shell pores are useful as supports for cell growth in tissue engineering for transporting nutrients to cells. Sponges have open shell pores. Closed shell pores are not interconnected, and liquid must permeate through a shell wall to enter another pore. It is possible to trap species in closed shell pores if said species are not able to permeate through the material that comprises the wall.
Vibrations in mechanical systems can have adverse consequences, such as fatigue, failure, and noise. Vibration suppression is achieved by passive or active methods. While active methods reduce vibrations in real time by making use of sensors and actuators, passive methods exploit the inherent ability of viscoelastic materials such as polymers to absorb and dissipate vibration energy.
Mechanical damping materials remove energy from a system. Motions to be damped can be periodic and regular (e.g., sine wave, square wave) or they can be irregular. Often a single mechanical event must be damped. Such an event is termed a shock, and the mechanical damping is termed shock absorption. Most damping measurements apply a periodic deformation to the article being tested, but it is also possible to assess the damping characteristics of a material from a single shock.
Damping or shock-absorbing properties are not determined from static measurements. Damping properties are ascertained by time varying or periodic deformation of the sample. Thus, a soft material (low E) is not necessarily a good candidate for damping. Furthermore, a material that is effective for damping over a certain frequency range may not be effective for damping over another frequency range. Therefore, in reporting a complex modulus (E* or G*), a frequency or frequency range is preferably specified.